Small molecules are chemical compounds with molecular weights below 900 Daltons. Most drugs are small molecules. The small size allows them to diffuse across cell membranes so that they can reach intracellular sites of action. In practice, small molecule compounds are usuallydissolved in DMSO or similar solvents, and are often poorly soluble in aqueous buffers at higher concentrations. For the preparation of serial dilutions using small molecule stock solutions in DMSO, refer to the article on DMSO.